Unusual dynamic relaxation of triplet-excited meso-phenyl-substituted porphyrins and their chemical dimers at room temperatures

It has been found that for mono- and di-meso-phenyl-substituted octaethylporphyrin (OEPs), etioporphyrins, corresponding Zn complexes and OEP chemical dimers (with a meso-phenyl spacer) a drastic shortening of triplet lifetimes from a ms to μs timescale in deaerated toluene solutions at 295 K takes place without any changes of the spectral-kinetic parameters of the S0 and S1 states. The observed effects are connected with torsion librations of the phenyl ring around a single C–C bond in sterically encumbered porphyrins leading to non-planar dynamic distorted conformations in excited T1 states accompanied by an enhancement of the T1-state nonradiative deactivation pathways.